Generation of electromagnetic waves by doppler effects



June 26, 1951 s. RAMO 2,558,001

GENERATION OF ELECTROMAGNETIC WAVES BY' DOPPLER EFFECTS Original Filed March 15, 1945 WET- byXMf/z) M His Atto ney Patented June 26, 1951 UNITED STATES; P TENT OFFICE GENERATION OF ELECTROMAGNETIC WAVES BY DOPPLER EFFECTS Simon Ramo, Los Angeles, Calif assignor to G eneral Electric Company, a corporation of New York Original application: March- 15; 1945,. Serial No. 582,916. Divided and this application June 12, 1948, Serial-N0. 32 ,615

1 This application is a division of my copending application, Serial No. 582,916. filed March 15; 1945. The present invention relates to the generation of electric waves and is concerned primarily with new and improved methods and means for generating waves of electric energy by the utilization of Doppler effects. I

It is known that a source of waves moving with respect to an observer appears to that observer to have a: frequency different from the emission frequency of this source by an amount dependent upon the velocity of the source relative to the observer. The frequency appears to b greater" magnetic waves of a given wave length to waves of a different. wave length, either shorter or longer.

It is a still further object of my invention to provide. new and improved methods and means for producing electromagnetic Waves having a length of the order of one millimeter.

It is a further object of my invention to provide new and improved. method v and means for producing extremely short waves at a minimum power loss.

It is. a still further object of my invention to provide a new and improved system for producing frequency modulated high frequency waves.

It is a still further object of my invention to provide a new and improved system for producing. frequency modulated high frequency waves which permits accurate control of the center frequency of the output waves.

It is a still further object of my invention to provide a new and improved method and means for producing frequency modulation of waves from sources of ultra high frequency waves which do not in themselves readily permit shifting: of

frequency.

It is a still further object of my invention to provide new and improved means and method for producing frequency modulated converted waves.

,Imitsbroad aspect, my invention consists in reflecting electromagnetic waves of a given f're-' 2 quency from a moving electric charge topro'duce another" electromagnetic wave of a different frequency, the difference in frequency between the two waves being a function of the relative velocities of the moving charge and the incident elec trcmagnetic wave. In another of its aspects, the invention contemplates production of extremely short waves by obtaining multiple reflections between moving charges and electromagnetic waves. In still another of its aspects, the invention consists in moving a modulated charge relative: to astationary wave transmitting: system to' produce an effect upon the output of the system which is a function of the movement of the charge.

For a better understanding of my invention, reference may be had to the following description taken in connection with the accompanying drawings and its scope will bepointed out'inthe appended claims. Fig. 1- of the drawing. illustrates basically the interaction of moving charges and electromagnetic waves to produce other electromagnetic waves by Doppler efiects; Fig. 2 illustrates schematically an ultra high frequency generator employing the principles of the basic illustration of Fig. 1; Fig. 3- shows a modificationof the invention as applied to the source of moving charges; Fig. e illustrates apparatus for producing electromagnetic waves by the interaction of a rotating space charge and-an electrowave.

In Fig; 1 there is shown a stationary source I which may be for example a probe or' other type of electrode for producing an electromagnetic Wave 2-,. the wave 2 being indicated as moving to the left in this figure. The numeral 3* represents a high speed electric charge moving in a direc-- tion opposite to that of theelectromagnetic wave 2'. I have discovered that the interaction of the:

electromagnetic waves 2 of a stationary source with a high speed charge 3 may be utilized toproduce a Doppler effect for generating an additiona1 electromagnetic wave. For maximum utilization of the theoretical possibilities of the effect, relatively high speed electron may be used.

The formula for describin the Doppler effects without makingany approximations is as f0110WS-2 where f and J" are the frequencies-observed by sour'ce, respectively.

where v is the relative velocity of the two observers and c is the velocity of light. The charge 3 may be for example a group of electrons, that is a region holding electric charge which is moving toward the source I with the velocity v, the source I being at rest and emitting an electromagnetic wave having a frequency as measured by a stationary observer as It. The frequency fe which the electrons of the charge 3 see is Accordingly, the electrons of the charge 3 are caused to vibrate around their average position because of the electromagnetic forces of the incident waves 2, the frequency of such a vibration having the value fe. In so vibrating, the electrons send out electromagnetic waves, these waves being a dispersion or reflection of the original impinging waves from the source I, of course. To an imaginary observer, however, who moves with the charge 3, the waves appear as being emitted with a frequency fe.

To an observer at the point I, the waves emitted from the source 2 appear as waves received from a moving source, the frequency of the received reflected waves being When Equation 2 is combined with Equation 3, the resultant value of the reflected wave is This is the value of the frequency of the reflected waves observed at the point I. It is apparent that to another stationary observer, at the point 4, for example, who is behind the group of moving electrons 3, the sign of B is reversed and this observer will receive waves dispersed from the source I at the original frequency f0, as well as waves generated by the vibrating electrons 3 due to the interaction between the electromagnetic waves 2 and the electrons 3. These newly generated waves have a frequency it which is less than the frequency of waves from the source 1. Since, by the utilization of sufiiciently high voltages in producing the high speed moving charge 3, it is possible to accelerate the electron charge to values at which B is close to unity, it is possible to obtain values of fr which are substantially greater than is. For example, if a wave emitted by the source i has a wave length of the order of one centimeter, by the use of a voltage of approximately 300,000 volts one obtains high speed electrons having a value of B of approximately .8 and through the interaction of such high speed electrons with the one centimeter wave, a one millimeter wave is dispersed from the electron.

Fig. 2 illustrates diagrammatically apparatus for producing extremely short waves in the manner explained above. A dielectric wave guide 5 formed by suitable conductive enclosing walls is supplied with electromagnetic waves having a frequency is from a source 6 through any suitable coupling means, such as a probe 1, the waves being indicated by the arrows 8. A beam of electrons is emitted from an electron gun 9, the gun having the usual heating element 10, and is directed into the wave guide 5 through an iris or filter ll used for reflecting the electromagnetic waves 8 to prevent their transmission through the open end of the wave guide 5.

4 The wave guide 5 functions likewise as the anode of a tube of which the gun 9 forms the cathode, accelerating potentials for electrons emitted from the gun 9 being supplied by a source of voltage, such as the battery l2. The electrons may be caused to enter the wave guide 5 as pulses of charge by the operation of a grid 13 and a high frequency modulating voltage Id. The most desirable frequency for the modulating voltage M from the point of view of efliciency of the electromagnetic wave generating system is a function of the dimensions and geometry of the wave guide, as well as the current density of the pulses of charge l5 and the accelerating voltage. It is apparent that, when many pulses exist in the wave guide at one time, a greater reflection of energy of the waves from the source 1 is obtained. It is important, however, that the wave guide 5 is not completely filled with a uniform space charge to prevent the medium within the guide from acting simply as a stationary medium of different dielectric constant. To this end, pref- V erably the modulating voltage I4 is of a fre-' quency high enough to insure variations in the electron beam before it travels the length of the guide. Stated in other words, the length of the guide 5 should be comparable with the length of the wave of the modulating source I4 if electrons having a velocity near the velocity of light are used.

As explained previously, because of the interaction of the electromagnetic forces of the impinging waves 8' on the electrons which form the pulse of high speed electric charge I 5, these electrons are caused to vibrate around their average position and disperse an electromagnetic wave having a frequency fr indicated by Equation 4. Such waves are indicated by the arrow 16. The generated waves [6 of very short wave length may be permitted to pass through the branch wave guide ll to an output guide l8 through the action of any suitable filter l9 tuned to the frequency of waves I6, the function of the filter 19 being to reflect the electromagnetic waves V 8 while permitting passage of the higher fre quency electromagnetic waves l6.

Means are also provided for frequency modulating the newly generated waves l6 and comprise means, such as the movable potentiometer arm 20, for varying the accelerating potential "applied to the pulses of charge which pass through the grid l3.

Another important feature of the system doscribed is that because the electromagnetic waves 8 in passing through the wave guide 5 are either length of wave guide 5 in itself being a relatively high Q circuit, that is, a circuit whose ratio of stored energy to dissipated energy is large. The energy of the wave guide 5 is either converted into new waves by reflection of the moving charges or reflected from the iris H and stored in the high Q circuit, an extremely small amount of energy being dissipated in the enclosing structure.

In the schematic illustration of Fig. 3, there is shown a modification of the principle outlined in the discussion of Fig. 1. In this modification, a

stationary source, such as the probe 2|, excited by the high frequency generator 22, emits electromagnetic waves 23, 24 of a given fre quency. Two movin charge pulses 25, 26 of different velocity characteristics approaching the source 2| act as reflectors for the waves 23, 24 to produce waves of higher frequency. The refiected waves of higher frequency 21, 28, in turn, impinge upon the moving charge pulses, or other charges of a beam of which the charges 25, 26 form a part, to produce other reflected waves having a still higher frequency. By such multiple reflection of waves between moving charges, waves of extremely high frequencies and very short wave lengths may be produced even though the pulses of moving charges 25, 26 have only moderate velocities.

Fig. 4 illustrates another modification of my invention in which electrons in a rotating space charge are used to create the moving reflector for incident electromagnetic waves. The rotating space charge 29 may be established, for example, within a magnetron having a centrally located cathode 30, an anode 3|, and a magnetic system 32 for producing rotation of the electrons emitted by the cathode 3B. The electrons preferably are produced in pulses by the action of a source of modulating voltage 33 applied between the anode 3| and the cathode 39. An input electromagnetic wave 34 is introduced into a wave guide section 35 connected to the magnetron tank 36 by any suitable input probe 31. When the wave 34 impinges upon the high speed electrons 29 rotatin in a direction opposite to the direction of motion of the wave 34, this wave is reflected to produce an electromagnetic wave 33 of higher frequency and shorter wave length. The wave 38 may be supplied to any suitable utilization means through a branch wave guide 39, filtering means 40 being utilized to permit passage of the wave 38 and reflection of the wave 34. In the same manner, the filter 4| at the end of the wave guide 35 may be used to reflect a shorter wave length 38 to prevent leakage of energy of this frequency at the end of the wave guide 35. It is understood of course that the filters 40, 4| are so placed with respect to the remaining portions of the wave guide structure that multiple reflections of both waves 34 and 38 within the wave guide system are prevented.

One of the advantages of the system of Fig. 4 is that relatively high efficiencies of operation may be obtained, since the electron beam 29 is maintained in rotation and a minimum number of electrons are collected by the anode 3|. In the ideal case of course no unidirectional currents are collected in the anode 3|. An alternative form of the principle of the system of Fig. 4 may be utilized in an electronic tube of the Barkhausen-Kurz type in which multiple oscillation of electrons is obtained with a consequent reduction of the unidirectional current required for operation of the system.

Among the important uses of the system illustrated are to obtain minute diiferences in frequency, such as are required in frequency analyzers, as well as to obtain scanning frequencies for use in such analyzers. The principles of the systems of Figs. 1-4 may be employed likewise for investigation of all types of gaseous discharges, such as a lightning discharge, the impingement of an electromagnetic wave upon the discharge being effective to produce a new electromagnetic wave of difierent frequency.

While I have shown and described my invention as applied to particular systems embodying various devices diagrammatically shown, it will be obvious to those skilled in the art that changes and modifications may be made without departing from my invention and I therefore aim in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A generator of ultra high frequency waves comprising means for establishing a rotating space charge, and means for impinging a beam of electromagnetic waves upon said charge to form by reflection therefrom electromagnetic waves having a frequency dependent on the relative velocity of said charge and said first waves.

2. An electromagnetic wave generator comprising a closed container, means for establishing a rotating space charge in said container, means for varying the instantaneous density of said space charge, means for reflecting an electromagnetic wave on said rotating charge to produce a reflected electromagnetic wave, and means whereby said reflected waves may be supplied to a utilization circuit.

SIMON RAMO.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,391,914 McElhannon Jan. 1, 1946 2,409,991 Strobel Oct. 22, 1946 

